An easy, efficient, and effective way to retrieve stored data is obviously one of the key issues of any information system. In the last few years, considerable effort has been devoted to the definition of more intuitive, visual-based querying paradigms, attempting to offer a good trade-off between expressiveness and intuitiveness. In this chapter, we analyze the main characteristics of visual languages specifically designed for querying information systems, concentrating on conventional relational databases, but also considering information systems with a less rigid structure such as Web resources storing XML documents. We consider two fundamental aspects of visual query languages: the adopted visual representation technique and the underlying data model, possibly specialized to specific application contexts.
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Traditional database systems are usually deployed in closed environments where users access the system only via a restricted network (e.g., an enterprise’s internal network). With the emergence of the World Wide Web, access has become possible from virtually anywhere, to any database with a Web interface. These Web-accessible databases, or Web databases, provide an elegant solution to store any data content to which a ubiquitous access is needed (Gribble, 2003). However, there is a need to provide users with a uniform, integrated view for querying the content of multiple Web databases.
Providing an integrated view of multiple databases is both important and challenging. Two particular challenges must be overcome: connectivity and interoperability. The Web has provided the necessary “pipes” to interconnect isolated data islands. However, to address the interoperability issue, more than a networking infrastructure is needed. The challenge remains to cope with the heterogeneity amongst the different databases as it obstructs interoperability. The need has therefore arisen for a middleware that transcends all types of heterogeneities and provides users with a uniform view of the content of Web databases (Bouguettaya, Rezgui, Medjahed, & Ouzzani, 2004).
In the context of Web databases, a middleware achieves uniform database access and interoperability. The challenge is to provide across-the-board transparency in order to allow users to access and manipulate data irrespective of platforms, locations, systems, or any other database-specific characteristics (Vinoski, 2002). To meet this challenge, we identify the following key issues:
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Locating relevant information sources. In Web applications, the information space is very large and dynamic. A way must be found to organize that information space in a rational and readily comprehensible manner to facilitate the location of pertinent data.
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Understanding the meaning, content, terminology, and usage patterns of the available information sources. Users must be educated about the information of interest and dynamically provided with up-to-date knowledge of database contents. Users must also be instructed as to the appropriate means of linking to information sources.
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Querying sources for relevant information items. Once appropriate information sources have been found, users need to be provided with the tools necessary to access and integrate data from these information sources.
To address the previously mentioned issues, we have developed the WebFINDIT system. The major contribution of the system is providing support for achieving effective and efficient data sharing in a large and dynamic information space. WebFINDIT presents an incremental and self-documenting approach. The system processes a user query in two steps. First, querying metadata for information sources location and semantic exploration. Second, querying selected sources for actual data. WebFINDIT provides support for educating the user about the available information space. The efforts related to registering and advertising the content of information sources are minimized.
We have provided an extensible middleware for querying autonomous Web databases and applications. We have incorporated Web services in our system to provide uniform access to applications. The Web services technology has been developed to assist in the integration and interoperation of isolated, autonomous and heterogeneous sources of information and services. The participants of a Web services system do not have to worry about the operating system, development language environment or the component model used to create or access the services.